During the Apollo 15 mission in 1971 and the Apollo mission in 1972, astronauts buried platinum thermometers at different depths in the lunar soil. The goal was to see how heat flowed from the interior of the moon out to the surface, which would help scientists learn more about the core and internal geologic activity. However, the thermometers unexpectedly reported a gradual uptick in temperature by one and a half to three and a half degrees celsius. Scientists couldn’t pinpoint why exactly. They were able rule out the Sun and the thermometer’s power generators. But there were still other possibilities, like fluctuations in the moon’s orbit, radiation from Earth, or maybe some lasting effect from the astronauts. But we did know which, because we were missing some crucial data.

Each of these probes beamed back temperature data for years. The data were recorded on 7-track magnetic tapes, which were then analyzed and archived. But when the experiment ended in 1977, only tapes up to 1974 had been analyzed and stored away. The rest from 1975 to 1977 were lost, and it wasn’t until 2010 that NASA started searching for old mission data from Apollo and other programs.

Researchers discovered 440 of them had somehow wound up at the Washington National Records Center. 440 tapes sounds like a lot, but it’s less than 10% of the missing data, and what was found was badly degraded. In another stroke of luck they found weekly logs of the probe’s temperatures during the missing years at the Lunar and Planetary Institute in Houston.

So, after eight years of searching and analyzing they finally had all the pieces of the puzzle.